The present invention relates to a resonant circuit comprising an induction element and a MOS (Metal Oxide Semiconductor) variable capacitance element connected in parallel with the induction element, and also relates to a voltage-controlled oscillator using such a resonant circuit.
FIG. 1 shows a circuit configuration of an LC resonance type voltage-controlled oscillator used in general. In this configuration, an LC resonant circuit 10 comprising nMOS transistors Q5, Q6 and inductance elements L1, L2 is connected to a first negative resistance circuit consisting of nMOS transistors Q1 and Q2, and also to a second negative resistance circuit consisting of PMOS transistors Q3 and Q4, wherein the voltage-controlled oscillator can be driven to oscillate at the resonance frequency of the resonant circuit. The MOS transistors Q5 and Q6 constitute a variable capacitance element whose capacitance value is varied by controlling a DC voltage applied to the gates of the MOS transistors Q5 and Q6. The drains and sources of the MOS transistors Q5 and Q6 are connected in common mutually and are connected to a control terminal Vtune. The capacitance can be varied by changing the DC voltage at the terminal Vtune through utilizing the characteristics of such MOS transistors, hence controlling the oscillation frequency of the oscillator.
[Cited patent reference 1]
Japanese Patent Laid-open No. 2002-43842
FIG. 2 shows the respective capacitances and inductances in the component parts of the oscillator shown in FIG. 1. Each of the PMOS transistors Q3 and Q4 has a capacitance Cgs_p+Cds_p corresponding to a sum of the gate-source capacitance and the drain-source capacitance thereof, as shown in the diagram. Each of the inductance elements L1 and L2 has an inductance L0. Each of the nMOS transistors Q5 and Q6 has a variable capacitance Cv0 between the gate and the drain-source and has a capacitance Cgs+Cds corresponding to a sum of the gate-source capacitance and the drain-source capacitance thereof, and also has a parasitic capacitance Cparasitic which is a capacitance to the ground due to wiring.
FIG. 3 shows an equivalent circuit representing the resonant circuit in the oscillator of FIG. 2. As shown in FIG. 3, the resonant circuit has an inductance L0 based on L1 and L2, a capacitance Cp0 composed of the parasitic capacitance derived from the wiring of the circuit and the capacitance incidental to the MOS transistors constituting the negative resistance circuit, a variable capacitance Cv due to the MOS transistors, and an equivalent parallel resistance Rp of the resonant circuit 10.
The equivalent circuit of FIG. 3 can be simplified as shown in FIG. 4, wherein:Cv=Cv0/2Lp=2·L0Cp=(Cgs+Cds)/2+(Cgs—p+Cds—p)/2+Cparasitic/2
FIG. 5 graphically shows an exemplary characteristic of the equivalent parallel resistance Rp and the variable capacitance Cv of the MOS transistors varied in accordance with the DC control voltage Vtune. As shown in this graph, the equivalent parallel resistance Rp changes sharply in response to the DC control voltage Vtune. And the inclination of the capacitance variation curve within the range of the control voltage is also steep. Particularly the equivalent parallel resistance Rp of the variable capacitance element affects the Q of the resonant circuit considerably, and the characteristic of the oscillation circuit is changed with a change of the Q value caused by the control voltage. Since the phase noise of the oscillator is rendered worse in general when the Q of the resonant circuit is low, the characteristic shown in FIG. 5 signifies that there exists a point where the phase noise characteristic of the voltage-controlled oscillator is extremely deteriorated depending on the frequency. In the example of FIG. 5, the deterioration of the phase noise resulting from a fall of the Q becomes maximum at the oscillation frequency generated by the control voltage to obtain the minimum value Rpmin of the equivalent parallel resistance Rp.
In the Cited patent reference 1, there is disclosed an LC resonant circuit having MOS type varactors of voltage-controlled variable capacitance elements connected in parallel, wherein staircase control voltages are generated by dividing the control voltage, and the staircase control voltages are applied respectively to the control electrodes of the plural MOS type varactors; and there is also disclosed a voltage-controlled oscillator employing such LC resonant circuit. According to this structure, any variation caused in the capacitance due to a change of the control voltage can be rendered gentler to consequently widen the allowable amplitude of the control voltage for the voltage-controlled oscillator.
However, according to this related art disclosed in the Cited patent reference 1, an output end of an external control voltage generator is grounded via a series connection of plural MOS transistors and resistors serving as a voltage drop means, and a staircase control voltage is generated from each midpoint of the connection. Therefore, in this structure, a current keeps flowing continuously from the control voltage generator into the voltage drop means. Generally, a charge pump circuit is employed as the control voltage generator. Accordingly, there exists a problem that the voltage drop means used for generating such staircase control voltages becomes a heavy load on the charge pump circuit. That is, ideally, a DC input impedance of the controlled circuit is desired to be infinite as viewed from the output end of the charge pump circuit, but a problem arises actually that such an ideal is not attainable in the known structure based on the above related art.